1. Field of the Invention
This invention relates to hot melt adhesive compositions which include a neutralized sulfonated 1,2-syndiotactic polybutadiene resin which has about 5 to about 50 meq. of neutralized sulfonate groups per 100 grams of the neutralized sulfonated polybutadiene, and about 25 to about 250 parts by weight of a hydrocarbon resin of a petroleum or coal tar distillate, having about 5 to about 6 carbon atoms the hydrocarbon resin being composed of aliphatic dienes and monoolefins per 100 parts by weight of the neutralized sulfonated 1,2-syndiotactic terpolymer.
Broadly speaking, synthetic adhesives used in packaging can be classified into four categories: water-based, solvent-based, reactive and hot melt adhesives. Of these four, currently the water-based are used most extensively. Usually, the water-based adhesives are based on emulsion polymers and are applied to porous cellulosic substrates. Energy from the outside in some fashion is applied to the system to evaporate the water in order that a strong bond may be formed. Beside this energy requirement for the formation of the bond, there is another complication with the use of water-based adhesive. For a uniform coating, a good uniform wetting of the substrate surface is desired, which is not easily achieved.
With the solvent-based adhesives, usually a good wetting is achieved; however, their use has been becoming extremely restrictive due to expensive energy requirements for the evaporation of organic solvents fire hazards associated with the use of these organic solvents and emissions problems with said solvents. The strict government environmental regulations and restrictions concerning worker's exposure to solvent vapors, etc., has placed extra pressure on the packager to use non solvent-based adhesives.
Recently, the use of hot melt adhesives has been growing very rapidly in the packaging industry. The hot melt adhesives are generally applied (as the name implies) by conventional extrusion or coating techniques in the temperature range of 250.degree. to 450.degree. F. on one of the surfaces to be bonded. The other surface is brought in contact with the hot surface for a sufficient period of time for the melt to cool, whereupon solidification of a strong and durable bond is formed.
The key requirements of resins suitable for hot melt adhesive applications are that they should have good physical properties, e.g., tensile strength, etc., at ambient conditions, and they can flow easily at fabrication temperatures. The ethylene vinyl acetate (EVA) copolymers and styrene block copolymers, such as Kraton, have been extensively used as hot melt adhesives; however, their use has been limited largely to pressure sensitive adhesives.
There is a significant demand in the market today for polymer systems which exhibit good green strength or tensile properties at ambient temperatures which, when heated to a predetermined temperature, will give good flow such that they may be applied to a coating or substrate by melt extrusion or other suitable techniques. In the past, it has been common to employ organic solutions as a way of applying a coating of a polymer system which is designed to have good adhesive properties. This practice is now under considerable pressure, due to the fact that the organic solvents must be evaporated from the coating giving rise to pollution problems and excess energy requirements.